How do you tell someone you don’t wanna talk to them anymore without it sounding mean?

Advise from an old friend of mine would be to tell them you slept with their sibling... and they'll never bother you again! (Joking).

But it's mean and their is no way around it, you either just got to say it or just leave them hanging...

I prefer to be told, though it hurts more in the moment, I'm not wondering why they're not talking with me anymore.